1. Field of the Invention
The present invention relates to a solid-state imaging device, and in particular to a solid-state imaging device structured so as to enhance the sensitivity of a light reception section thereof and a method for producing such a device.
2. Description of the Related Art
In recent years, the transmission and reception of images is becoming indispensable to portable information devices and the like including cellular phones. So-called solid-state imaging devices such as CCDs are used for imaging purposes while liquid crystal panels are used for displaying images. In the field of solid-state imaging devices, CMOS image sensors based on a so-called CMOS logic process, which is typically used for producing a normal integrated circuit, are widely developed with a view to achieving low power consumption and a low cost. As in the case of CCDs, the demand for higher resolution and miniaturization has necessitated CMOS image sensors having reduced pixel size, as well as having a reduced area of the light reception section, and a reduced aperture size in a light shielding film. However, since CMOS image sensors have so far been developed based on the so-called logic process, very little attention has been paid to the optical characteristics of CMOS image sensors, such as reflection or refraction at interfaces between the layers of multi-layered films incorporated therein. Therefore, it is difficult with CMOS image sensors to efficiently converge incident light and obtain sufficient sensitivity.
A conventional solid-state imaging device, in particular a CMOS image sensor, will be described with reference to FIG. 5. A light reception section 12 is formed on a top surface of a silicon substrate 11 for converting incident light (hν) to an electric charge. An interlaminar insulation film 13 is formed on the silicon substrate 11 for electrically isolating a first metal layer 18, a second metal layer 19, and a light shielding film 14 from one another. The light shielding film 14 is formed so as not to overlie a light reception face of the light reception section 12, so that light incident on the interlaminar insulation film 13 does not fall anywhere except the light reception section 12. A passivation film 15 is formed on the light shielding film 14 and on the interlaminar insulation film 13 which is present within an aperture of the light shielding film 14. The passivation film 15 provides moisture-resistance, chemical resistance, and improved barrier properties against impurities such as Na ions, oxygen, etc., and metals. A planarization film 16 is formed on the passivation film 15. A microlens 17 is formed for converging light which is incident on the planarization film 16. As the interlaminar insulation film 13, a deposited film such as a P (plasma CVD)-SiO2 film, an NSG film (a silicon oxide film containing no impurities), a BPSG film (a silicon oxide film containing phosphorus and boron) or the like is used. As the passivation film 15, a monolayer film, such as a P (plasma CVD)-SiN film, or a deposited film composed of a P-SiN film and a PSG film (a silicon oxide film containing phosphorus), is generally used. The planarization film 16 is generally composed of an acrylic material. In the case of a color solid-state imaging device, an acrylic material and a color filter are utilized as the planarization film 16. In conventional CMOS image sensor structures, the passivation film 15 has a stepped portion as described above, which prevents the convergence of light. When such a structure is adapted for higher resolution and miniaturization, the stepped portion of the passivation film 15 causes a decrease in the amount of light converged on the light reception section 12, thereby making it difficult to obtain sufficient sensitivity.
A method for producing the above-described device will now be described with reference to FIG. 6. The light reception section 12 is formed in the silicon substrate 11 through ion implantation, heat treatment, etc. After forming a polycrystalline silicon film and a silicide film on the silicon substrate 11 by using a CVD technique, a gate electrode (not shown) is formed by patterning, etching, or the like. Thereafter, a BPSG film is deposited as the interlaminar insulation film 13 by a CVD technique. When the BPSG film receives heat treatment at a high temperature, the film becomes fluid so that its surface can be flattened. By taking advantage of such characteristics of the BPSG film, the surface of the BPSG film is flattened so as to facilitate the formation of the first metal layer 18. The first metal layer 18 is formed by depositing TiN, Al or the like on the BPSG film by using a sputtering or CVD technique. Upon the first metal layer 18, the P-SiO2 film is deposited as the interlaminar insulation film 13 by using a CVD technique and the surface thereof is flattened by chemical machine polishing. Thereafter, as in the case of the first metal layer 18, a thin film of TiN, Al, or the like is formed as the second metal layer 19 by using a sputtering or CVD technique. Similarly, upon the second metal layer 19, a P-SiO2 film is deposited as the interlaminar insulation film 13 by using a CVD technique and the surface thereof is flattened by the chemical machine polishing. Thereafter, TiN, Al, or the like is deposited as the light shielding film 14 by using a sputtering or CVD technique, and the resultant film is patterned and etched so as not to overlie the light reception section 12. Upon the light shielding film 14, a P-SiN film is deposited as the passivation film 15 by using a CVD technique or the like. The planarization film 16 is formed by applying an acrylic material to the passivation film 15. In the case of a color solid-state imaging device, an acrylic material is applied; a color filter is formed; and then the acrylic material is further applied thereto as a protection coating, thereby completing the planarization film 16. Thereafter, a lens material is applied and the microlens 17 is formed by patterning and heat treatment.
In a conventional solid-state imaging device shown as FIG. 5A, the refractive index of the P-SiN film used for the passivation film 15 is about 2.0 while the refractive index of the acrylic material used for the planarization film 16 is about 1.5 to 1.6. In the case where the refractive index of the passivation film 15 is higher than that of the planarization film 16, when light falls onto an edge of the passivation film 15, the incident light is not converged on the light reception section 12 but rather refracted so as to travel outside the light reception section 12 toward the first metal layer 18 or the second metal layer 19 since the edge of the passivation film 15 has a rounded shape. As shown in an enlarged view of FIG. 5B, when light falls onto the flat top surface of the stepped portion, total internal reflection can occur at an interface between the planarization film 16 and a face of the passivation film 15 parallel to a side face of the light shielding film 14, depending on the incident angle of the light, since the refractive index of the passivation film 15 is higher than that of the planarization film 16. The incident light passes along the face of the passivation film 15 parallel to the side face of the light shielding film 14 so that the light is not converged on the light reception section 12. As described above, in the conventional structure shown as FIG. 5A, any light incident on a portion of the passivation film 15 neighboring the side face of the light shielding film 14 is not converged on the light reception section 12. Therefore, the effective aperture size of the light shielding film 14 is smaller than the actual aperture size by an amount corresponding to the thickness of the passivation film 15.
The light reception face of the light reception section 12 used to be sufficiently large relative to the stepped portion of the passivation film 15. However, due to the reduced pixel size which is necessitated for improved resolution and miniaturization, the light reception face of the light reception section 12 is becoming smaller and the aperture of the light shielding film 14 is also becoming narrower. Therefore, the ratio of the amount of light incident on the stepped portion of the passivation film 15 to the amount of light incident on the aperture of the light shielding film 14 increases, thereby making it difficult to obtain sufficient sensitivity. However, the passivation film 15 can not be omitted because the passivation film 15 plays an important role in providing moisture-resistance, chemical resistance and/or improved barrier properties against impurities such as Na ions, oxygen, etc., and metals.
The effect of the stepped portion of the passivation film 15 on the numerical aperture of the light shielding film 14 will now be described in detail. Provided that the sensitivity deteriorates by an amount corresponding to the thickness of the passivation film 15, the aperture size can be represented as (pixel size)−(width of the light shielding film)−(2×thickness of the passivation film 15). For example, in the case where the width of the light shielding film 14 is 1.5 μm, and the thickness of the passivation film 15 is 0.5 μm, given a pixel size of 10 μm×10 μm, the aperture of the light shielding film 14 is calculated to be 10−1.5−(0.5×2)=7.5 μm, and therefore, the numerical aperture is 75%. Similarly, given a pixel size of 5 μm×5 μm, the aperture of the light shielding film 14 is calculated to be 5−1.5−(0.5×2)=2.5 μm, and therefore, the numerical aperture is 50%. Starting from the above one-dimensional calculation, it will be seen that the numerical aperture area as calculated in a two-dimensional manner gives rise to an even greater difference in the size (area) between the aperture and each pixel. The reduction in the numerical aperture of the light shielding film 14 is highly detrimental to the ratio of light incident on the light reception face of the light reception section 12; this effect is more detrimental than any reduction in the numerical aperture of layers below the light shielding film 14.